CYTOSKELETON-MEMBRANE INTERACTIONS IN THE CNIDOCIL COMPLEX OF HYDROZOAN NEMATOCYTES

Each cnidocil complex of the hydrozoans Tubularia larynx and Hydra vulgaris consists of 9 or 7-10 large stereovilli (= stereocilia), respectively, and a modified cilium, the cnidocil. The cnidocils comprise the regular 9 microtubule doublets, up to 30 additional microtubules, as well as a central filament body. Adjacent stereovilli are linked together by intermembrane connectors forming the stereovillar cone. The distal tips of the stereovilli surround the cnidocil in a closed tubular arrangement measuring up to 0.7-mu-m in length. Within this contact region the cnidocil is linked to the stereovillar tube by another set of intermembrane connectors, which seem to hold the cnidocil in a central position within the stereovillar cone. Stereovillar membrane and actin core are linked by 16-nm long cross bridges, which display a periodicity of 16 nm and emerge from the actin core. Within the cnidocils periodically arranged membrane-cytoskeleton bridges are uniformly restricted to the contact region. Here, 24-nm long cross bridges, which are spaced by a regular distance of 20 nm, interconnect the A-tubules of the microtubule doublets and the membrane The cnidociliary membrane is differentiated into distinct domains as revealed by freeze-fracturing. Within the contact region of the nematocytes of Tubularia larynx, intramembrane particles are arranged in 9 rows of 700 nm length and 50 nm width, separated by particle-free areas. Intramembrane particle are irregularly distributed distal to the contact region. Considering recent physiological results we presume that the latter represent chemoreceptor units, while mechanical stimuli are transmitted via the intermembrane connectors and the microtubule-membrane bridges to mechanosensitive channels within the domain of the cnidociliary membrane in the contact region.